Add segmentation + classification head fine-tuning example

[qubvel]

It would be great to have more examples for fine-tuning in the library!

Similar to current examples for binary-segmentaion and multi-label segmentation would be great to have example for segmentation + classification head. The same camvid or oxford pet dataset can be used. As a classification target for these datasets, the "existence" of the class on the image can be used.

The example should showcase how to

• fine-tune a model with pytorch-lightning (or any other fine-tuning framework, even a plain pytorch)
• use loss function for segmentation and classification
• compute metrics for classification and segmentation heads
• visualize results

Docs on how to create a model with classification head.

In case anyone wants to work on this you are welcome! Just notify in this issue, and then ping me to review a PR when it's ready.